The present invention relates to an optical head and method for focusing an optical beam on an information recording medium such as an optical disk, to record information on, reproduce it from, or erase it from, the information recording medium and, more particularly, to a system for detecting the displacement of the focal point of the optical head.
Recently, various systems for recording information on an information recording medium, e.g., an optical disk, have been developed. In each system, a beam spot corresponding to the waist of a light beam converged by an objective lens of an optical head must be formed on the optical disk to record information on, reproduce it from, or erase it from, the optical disk. The optical head has a system for detecting the focusing condition of the objective lens. A focusing condition detecting system which employs a so-called "knife edge method" is disclosed in U.S. Patent Application Ser. No. 399,873 filed on July 19, 1982 by the same assignee as the present application, corresponding to EPC Application No. 82106508.3 filed on July 19, 1982, now U.S. Pat. No. 4,521,680. The detecting system has a light shielding plate, provided between the data recording surface of an optical disk and a photodetector having two photosensing regions, to shield a part of a light beam reflected from the data recording surface and to allow the passage of another part of the light beam. The remaining part of the light beam is applied to the photosensitive regions of the photodetector. When the light beam is accurately focused on the optical disk, the photosensing regions generate electrical signals of the same level. When the beam is inaccurately focused on the optical disk, the photosensing regions output electrical signals of different levels. It can thus be determined from these signals whether or not the light beam has been accurately focused on the optical disk.
The accuracy of determining the focusing condition is proportional to the lateral magnification of the beam spot formed on the photosensor, which corresponds to the waist of the light beam which is formed on the optical disk. As long as the same objective lens having a fixed focal distance is used, the lateral magnification is inversely proportional to the angular aperture of the projection lens used to converge the light beam on the photodetector, and directly proportional to the focal distance of this projection lens. Hence, to raise the accuracy of determining the focusing condition, it suffices to use the focusing lens having a long focal distance. However, the longer the focal distance, the longer the light path of the optical system of the optical head, and the larger the optical head.